1. Field of the Invention
The present invention relates to an improved method of determining sweep and effective permeabilities of fluids flowing in an underground formation and, more particularly, to such a method which utilizes one or more gamma ray emitting tracers which are logged by the same logging tool.
2. Setting of the Invention
During a secondary or tertiary oil recovery project a displacement fluid or fluids are injected into the formation through an injection well and oil in the formation is displaced towards an offset production well. The efficiency with which the injected fluid contacts the oil bearing zones is termed "sweep efficiency". The percentage of available oil displaced by the injected fluid is termed "displacement efficiency". Both parameters, "sweep efficiency" and "displacement efficiency", are important in evaluating the displacement process and in obtaining a description of the reservoir. Various individual methods have been utilized to determine sweep efficiency of a waterflood project, and various individual methods have been utilized to determine the effective permeabilities of the swept formation. One such method used in the past involves injecting a first tracer fluid into the formation along with the injection fluid and at some later time injecting a second tracer fluid into the formation. A logging device run through an observation well would indicate the presence of the tracer fluids as they pass through the formation. However, experience has shown that many months after breakthrough of a tracer into a particular zone, a high background radiation of the first tracer may still be present. Thus, it has been extremely difficult to distinguish between the old tracer and the new, although an approximate evaluation could be made. If this evaluation was attempted with tracers injected several weeks apart, it would be virtually impossible to distinguish between the tracers at all. Thus, there is a need for a method of utilizing at least two radioactive tracers which can be distinguished.